What's new
What's new

Clearance for laser-cut tubing thru-holes?

bedinger57

Plastic
Joined
Apr 9, 2018
Hi guys,

This is my first post here but I have long since lurked(with no username).

I recently started as a CAD Drafter and I am reverse engineering a tubing weldment so that we can have it CNC lasercut. This will be welded to a piece of machinery, and has thru holes to bolt other pieces to it. I was just looking for some guidance as far as what kind of clearance would be necessary for bolt holes.

The bolts used will be 3/8 cap screws. I was looking to use .39(.015 clearance)but my boss is very skeptical of this number and I don't want to be the new guy who gets 500 pieces made and then has to drill out every hole. I know .39 would be fine for flat plate but as this is tubing I know if each hole is slightly off depending on tolerances you could lose all clearance.

Sorry if this is unclear, it's nearing 5oclock and I'm dreaming of afterwork beers.

So....
1: What is a safe clearance for tubing thru holes? Don't want a ton of slop if possible.

2: I'm unfamiliar with tubing lasers, for thru holes will it actually pierce all the way thru the tubing or will it pierce one wall, rotate the work piece and pierce the opposite wall?

Sorry if this has been discussed before, I'll again pull the new card. TIA!
 
It's gonna vary with different suppliers, and different size holes.

Work with the suppliers, and see what they can do.
 
For structural steel work, the standard bolt hole size is 1/16" larger than nominal bolt size. An "oversized" hole for structural steel is 3/16" larger than nominal bolt size (reducing to 1/8" larger for the smallest common structural bolt of 1/2"). And the allowance for steel-to-concrete anchor bolt holes is just gross.

For laser cutting, even if your tubing twists like a soda straw, I'd think the 1/16"-over would be a very generous upper bound on the fit allowance. And since you're talking about 3/8" cap screws, probably too generous. I'd look at Machinery's Handbook for free and loose fits for 3/8", and choose one or the other. Loose if you're paranoid about pretzeled tubing or misaligned cuts, and free otherwise.
 
You didn't mention the CAD software you're using, but I'll reference SolidWorks because that's what I'm familiar with. I mention this because the "Hole Wizard" has clearance holes for bolts and lets you specify (from memory) loose or tight fits. I'm a believer in proportional clearances and allowances, which is to say that you can't just give 1/32" clearance for all bolts from stupidly tiny to crazy huge and expect things to work out well. I suspect there's a chart, probably in Machinery's Handbook. I usually use the numbers from SolidWorks and don't think much more about it.

I do think you're being overly optimistic with your 0.015". The operators of these machines can be highly untrained and often don't bother to tweak the kerf offsets properly. I've seen 0.005 to 0.010 used up just with that negligence.
 
To answer your #2, the beam on sheet/tube lasers is highly focused near the surface of the material. That's where the heat is concentrated. If the tube size is small enough and the power set high enough, you might get some burning through, but it won't be pretty. So, yes, the tube is rotated and cut from different sides. This can cause some issues with alignment of the holes to the tube on some machines if the tube dimensions aren't the best.

You can design some pretty neat parts taking advantage of this. Many machines have a tilting head so that non-surface-perpendicular cuts can be made as well.

What you will get is a bunch of slag inside the tube, unless they have a decent way to get rid of it during cutting. Some use vacuum at one end of the tube, but in my experience there's still a lot of manual work to clean it out. If they aren't doing that, you receive a mess to deal with.
 
Most tube laser companies will quote +/-0.010" running at full speed and +/-0.005" if they slow down for maximum accuracy...safer to assume you will get +/-0.010".
 
My experiance with laser cutting is the process will reliably hold +-2 thou if the operator gets off there ass and dials it in at least say under 1/4" thickness. Your tube is probably the bigger variable. ERW i oftern find is anything upto +15 thou over and not round bellow even 1" dia. circular hollow sections are often far far worse too, not just in diameter but in circularity too.

Hell pretty much any tube other than CDS tends to vary a lot in round and overall diameter, this makes fits like your trying to achieve a gamble. Too tight and you lose a lot of time grinding them out, too lose and you lose all the alignment advantages. Come back and post up what tube and what thickness its going through and we can all give you the numbers we use, but me, my allowances change depending on job size material thickness and the tube dia. Also falls back on what pre-scion levels you truly need with your alignment, tube often is not as straight as you wished it was too!
 
Appreciate all the responses guys.

Yes I am using Solidworks, I didn't even think about using the default Hole Wizard functionality.

This is for agricultural machinery so definitely tighter tolerances than steel construction, but nothing near aircraft precision. I think I've settled on .395 as it is just underneath a .397 "free fit" and over the .386 "close fit".
 
Whatever you design it at, it's the tolerance that they'll have to hold. I suggest talking with the tube laser place and asking what they'll commit to. Set your base dimension to the smallest you want and set the tolerance +X/-0, where X is the deviation given by them. You're MUCH more likely to be ok with a hole that's bigger, but smaller holes will be a big problem.
 
A 3/8" bolt will fit in a 3/8" hole, so just take 0.375", add the laser tolerance (I hear 0.01" for tubing but for plate we hold 0.002" without trying on an old machine) and then add double whatever the paint / powder coat thickness will be (You didn't mention coating but it's an important concern). Note that the thickness inside holes can be substantially more than the thickness on the flat portions. (For small holes in powder coated steel I add 0.012", which is four times the nominal coating thickness). That will give you your minimum safe size for a single bolt, which is good enough for everything I do.

You end up with a much more complicated process if bolts are across multiple bends, but with tubing that shouldn't be an issue. The only time I have ever had a bolt not fit in a hole on a simple interface is when the hole size was very near the material thickness, and even then only in aluminum.


But the easiest thing to do is open your calipers to 0.4" or something, slip a bolt in there, and feel how much it wiggles around and if that will be an acceptable fit.



And the laser will only cut very near the head; too far from the head will be out of focus and too far from the nozzle for assist gas to work properly, so they have to cut the tube from both sides.
 








 
Back
Top